Multi-helix rotating shield brachytherapy (RSBT) applicator and multi-source RSBT apparatus are two novel intensity-modulated brachytherapy techniques for the treatment of cervical and prostate cancer, respectively. The use of imaging techniques such as magnetic resonance imaging guided brachytherapy has enabled the precise identification and contouring of tumor volumes for treatment planning, as well as demonstrated the challenges associated with using conventional high dose rate brachytherapy (HDR-BT) approaches to conform the radiation dose to the target and avoid surrounding sensitive healthy tissues. The target conformity of conventional HDR-BT dose distributions is restricted based on the geometrical constraints imposed by the position and shape of the tube-shaped applicators, as well as the radially-symmetric radiation dose distributions produced by the radiation sources. Dose distribution conformity for cervical and prostate cancer can be significantly improved relative to conventional HDR-BT through the use of multi-helix and multi-source RSBT techniques, respectively. In this study, two novel RSBT concepts for treating cervical and prostate cancer were introduced and the dosimetric impact was evaluated.
A Henschke-type cervical cancer applicator, designed for an electronic brachytherapy (eBx) source (Xoft AxxentTM) and a 0.5 mm thick tungsten partial shield with 180° or 45° azimuthal emission angles, is proposed. The interior wall of the applicator contains six evenly-spaced helical keyways that rigidly define the emission direction of the partial radiation shield as a function of depth in the applicator. The shield contains three uniformly-distributed protruding keys on its exterior wall and is attached to the source such that it rotates freely, thus longitudinal translational motion of the source is transferred to rotational motion of the shield. RSBT treatment plans were generated for five cervical cancer patients with a diverse range of high-risk target volume (HR-CTV) shapes and applicator positions. Treatment delivery time and tumor coverage (D90 of HR-CTV) were the two metrics used as the basis for evaluation and comparison.
With multi-source RSBT apparatus, precise angular and linear positioning of partially-shielded 153Gd brachytherapy sources in interstitial needles for the treatment of locally-advanced prostate cancer is carried out. Following needle implantation through the patient template, an angular drive mechanism is docked to the patient template. Each needle is coupled to a multisource afterloader catheter by a connector passing through a shaft. The shafts are rotated about their axes by translating a moving template between two stationary templates. Shafts’ surfaces and moving template holes are helically threaded with the same pattern such that translation of the moving template causes simultaneous rotation of the shafts. The catheter angles are simultaneously incremented throughout treatment. For each rotation angle, source depth in each needle is controlled by a multisource afterloader, which is proposed as an array of belt-driven linear actuators, each of which drives a wire that controls catheter depth in a needle.
In conclusion, the helical RSBT approach for treating cervical cancer and the multi-catheter RSBT approach for treating prostate cancer, powered with novel radiation sources amenable to shielding, are clinically- and mechanically-feasible techniques that dosimetrically outperform conventional brachytherapy methods while minimizing damage to healthy tissues inside and/or adjacent to the target.